Fig. E: You can home-ferment in an oven with a worklight
for heat. Fig. F: If your oven is too warm, you can make
a fermenter from a cardboard box lined in foil; a lightbulb
is suspended from the lid.

Fig. G: Every 6 hours, remove the foil and inspect your
beans. Finished beans will look dried and wrinkled.
Fig. H: You should see telltale white, sticky strings when
you lift a spoonful up from the mixture.

4. Inoculate.

Pour the beans into the baking dish. Mix in 2– 3
Tbsp of commercial natto, and spread the beans
evenly across the dish (Figure D). Carefully cover
with a sheet of foil, and punch air holes in the
surface. Some recipes suggest adding sugar or salt,
or a bit of the bean water if the mixture seems dry.
Mine turned out fine without any of these additions.

5. Ferment.

The natto bacteria must now grow at about 104°F.
If you have a modern oven, place the baking dish
on its top shelf, next to your thermometer. Put the
work light on the oven floor, turn it on, and close the
door (Figure E). Every hour, check the temperature.
You may have to change the wattage of your bulb or
prop the door open to get it right. Don’t worry if the
temperature fluctuates; I’ve hit as high as 122° and
as low as 95° before leveling out at 104°.

If, like me, your oven runs too hot or you can’t
trust your roommates to leave it alone, use a cardboard box or a cooler instead (Figure F). Or adopt
the style of Yoko Kondo, who ferments and pickles
food for Minako Organic, her Japanese restaurant
in San Francisco. Ever since she was a student in

Japan, Kondo has wrapped her foil-covered baking
dish in 3 bath towels and left it out to ferment for a
full day in direct sunlight. In the winter, she puts the
package in her bed under the covers.

6. Test.

Every 6 hours, remove the foil and inspect your
beans (Figure G). Finished beans will have dried,
darkened, and wrinkled on the surface, and should
give off a light aroma of ammonia. Pull up a spoonful with a sterilized spoon, and you should see the
telltale white, sticky strings (Figure H). If you haven’t
achieved these results after about 24 hours, something likely went wrong. Cover and store successful
natto in the refrigerator for 1– 2 days to let its flavor
develop. It will last for 2 weeks in the fridge, and the
bacteria can survive frozen for 2 months.